Keep track of whether a class is abstract or not. This is currently only used for the __is_abstract type trait.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67461 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Parse/ParseExpr.cpp b/lib/Parse/ParseExpr.cpp
index f2892c1..b140046 100644
--- a/lib/Parse/ParseExpr.cpp
+++ b/lib/Parse/ParseExpr.cpp
@@ -768,6 +768,7 @@
   case tok::kw___is_enum:
   case tok::kw___is_union:
   case tok::kw___is_polymorphic:
+  case tok::kw___is_abstract:
     return ParseUnaryTypeTrait();
 
   case tok::at: {